The performance and durability of electric machines can be directly related to the efficiency of the cooling system. Internal resistance increases due to some increased temperature can substantially decrease performance, and for some interior permanent magnet machines can substantially increase the risk of demagnetization. Further, some polymer-based insulation systems can be limited by cumulative damage largely due to elevated temperature operations. As a result, the higher the temperature, the shorter the life of the insulation systems. Maximizing heat extraction capabilities in a cost-effective fashion can extend machine life span.
Some electric machines were originally designed as oil-cooled devices, because of their potential use in transmission applications. Some electric machines can have more-varied applications and restrictions in terms of cooling and complexity. Some of the power electronics of the electric machines can be water-cooled. Also, in some applications, internal combustion engines also can be water cooled. The addition of a separate oil circulation cooling system for the electric machines can give rise to incompatibility with some applications.
In some applications, coolants of high dielectric strength, such as oil, can be problematic. However, for some applications, coolants of high dielectric strength can be preferred for some applications because they can provide a thermally efficient and cost effective solution for heat rejection from the internal components of some electric machines.